LAB Series L5 amp: RCA transistor equivalent?

[Richard Rasker]

Hi all,

I've got a rather old beastie here, a LAB Series L5 amplifier, apparently
capable of some 150 watts of output power, with a shorted end stage
transistor.

The transistor is an RCA type, with the following markings:

254 002C
RCA
CW 7923

The last number no doubt indicates the production year and week of the
transistor, but I can't find anything about this 254 002C type number.

Does anyone have a clue what the equivalent may be? The problem is that
there's ten of these in all, and proper repair procedure requires that I
replace them all, so "simply trying" a few likely types (such as the
MJ150XX series) isn't really a economical option.

Thanks in advance, best regards,

Richard Rasker

 

[N Cook]

Hi all,

I've got a rather old beastie here, a LAB Series L5 amplifier, apparently
capable of some 150 watts of output power, with a shorted end stage
transistor.

The transistor is an RCA type, with the following markings:

254 002C
RCA
CW 7923

The last number no doubt indicates the production year and week of the
transistor, but I can't find anything about this 254 002C type number.

Does anyone have a clue what the equivalent may be? The problem is that
there's ten of these in all, and proper repair procedure requires that I
replace them all, so "simply trying" a few likely types (such as the
MJ150XX series) isn't really a economical option.

Thanks in advance, best regards,

Richard Rasker

could it be 40254 , Ge, PNP, TO3, 32V, 5A

Case style and .2V or .5V for VBE and NPN/PNP of any isolated/pulled working
ones would be useful

 

[Richard Rasker]

Op Sun, 06 May 2007 14:01:06 +0100, schreef N Cook:

could it be 40254 , Ge, PNP, TO3, 32V, 5A

Case style and .2V or .5V for VBE and NPN/PNP of any isolated/pulled working
ones would be useful

Thanks for your reply but no, it's definitely a silicon NPN tansistor:
VBE=0.79V, VBC = 0.77V. (And I wouldn't expect germanium transitors to be
used in amps from 1979.)
And my apologies for omitting important data; it's indeed a TO3 case;

Closer inspection of the end stage reveals that there's eight (not ten) of
these, in two groups of four, each group driven by an RCA410 transistor.
The '254 transistor emitter resistors have a surprisingly high value:
3.9 ohms each, at approximately 3 watts.
The power supply is some +/-55 volts, which would indeed produce around
150 watts maximum power in an 8 ohms load.


Richard Rasker

 

[[email protected]]

Op Sun, 06 May 2007 14:01:06 +0100, schreef N Cook:





Thanks for your reply but no, it's definitely a silicon NPN tansistor:
VBE=0.79V, VBC = 0.77V. (And I wouldn't expect germanium transitors to be
used in amps from 1979.)
And my apologies for omitting important data; it's indeed a TO3 case;

Closer inspection of the end stage reveals that there's eight (not ten) of
these, in two groups of four, each group driven by an RCA410 transistor.
The '254 transistor emitter resistors have a surprisingly high value:
3.9 ohms each, at approximately 3 watts.
The power supply is some +/-55 volts, which would indeed produce around
150 watts maximum power in an 8 ohms load.

Richard Rasker

All outputs the same type as in quasi-complementary? Phase Linear ran
RCA410s into XPL909 which I think was a re-badged RCA423. It wasn't a
great transistor but was about the best there was in the early '70s.
Are you sure the emitter resistors are 3.9 and not 0.39 ohm? Values
from 0.1 to 0.47 were very common. It was very unusual to blow more
than 2 devices in a channel - 1 in each power supply leg that
'protected' the remaining devices - so could you run one of the
remaining devices into a curve tracer to get an idea of the
parameters?

I used to run 30KHz at about 1 watt to set the bias set for crossover
distortion. Very obvious on a scope. Those were the days.....

GG

 

[Richard Rasker]

Op Wed, 09 May 2007 22:20:35 -0700, schreef stratus46:

All outputs the same type as in quasi-complementary? Phase Linear ran
RCA410s into XPL909 which I think was a re-badged RCA423.

Hm, as far as I can see, an MJ15003 might be a reasonable replacement.

It wasn't a great transistor but was about the best there was in the
early '70s. Are you sure the emitter resistors are 3.9 and not 0.39 ohm?

I know, I thought it was very high too. And perhaps the silver ring has
picked up a bit of a brown tarnish over many years of use - I'll check it
out as soon as I get round to working on this beastie again.

so could you run one of the remaining devices into a curve tracer to get
an idea of the parameters?

The remaining transistors seem pretty ordinary, and don't appear to be
precisely matched - or if they ever were, this is no longer the case.
In particular, Vbe varies from 0.7 to 0.85 volts between separate devices,
when fed with a constant current (10mA). Perhaps this is a clue as to why
just one transistor failed.

I used to run 30KHz at about 1 watt to set the bias set for crossover
distortion. Very obvious on a scope. Those were the days.....

Yup, that's a nice way of getting it right - although I also keep an eye
on the absolute current. And when repairing blown end stages, the first
tests are usually performed with a 75-150W light bulb in series with the
mains supply. If anything causes a sudden current increase in the end
stage, the lamp will often prevent real trouble.

Anyway, thanks for your information, and I'll let you know how things
work out.

Richard Rasker

 

[[email protected]]

Op Wed, 09 May 2007 22:20:35 -0700, schreef stratus46:

Hm, as far as I can see, an MJ15003 might be a reasonable replacement. 0.39 ohm?

I know, I thought it was very high too. And perhaps the silver ring has
picked up a bit of a brown tarnish over many years of use - I'll check it
out as soon as I get round to working on this beastie again.


The remaining transistors seem pretty ordinary, and don't appear to be
precisely matched - or if they ever were, this is no longer the case.
In particular, Vbe varies from 0.7 to 0.85 volts between separate devices,
when fed with a constant current (10mA). Perhaps this is a clue as to why
just one transistor failed.


Yup, that's a nice way of getting it right - although I also keep an eye
on the absolute current. And when repairing blown end stages, the first
tests are usually performed with a 75-150W light bulb in series with the
mains supply. If anything causes a sudden current increase in the end
stage, the lamp will often prevent real trouble.

Anyway, thanks for your information, and I'll let you know how things
work out.

Richard Rasker

When I repaired consumer audio back in the early ' 70s, a real quick
check was to turn the amp on without the power transistors and run it
with just the drivers. It wouldn't deliver any power but it would make
the power supply rails so you could see it the remaining system worked
before you went to the trouble and expense of installing the new
output devices. Then run it at low power at 30 KHz and finish it up.

I agree on the MJ15003 but I suspect those are much tougher
transistors than what failed. I would bet you could do just fine with
half as many 15003s as the originals and still have more output
current capability as the original setup. It would however alter any
overload detection circuitry. Heck, load 'em all up.

Definitely interested in the rest of your story.

GG